Within a telecommunication system, it is possible to apply a flexible subframe scheme in order to better adapt the number of UL and downlink (DL) subframes to current traffic conditions. E.g. when there is mostly DL traffic, it is preferred to use a scheme with many DL subframes.
Flexible Subframes in Dynamic Time Division Duplex
In a dynamic Time Division Duplex (TDD) system, a group of subframes are fixed subframes, i.e. they are either UL or DL subframes in all radio frames, while others are flexible subframes, i.e. in some radio frames they can be UL subframe, while in other radio frames same subframe can be DL subframes or even special subframes. The assignment of the UL or DL direction is done in a dynamic manner on the basis of a radio frame or multiple radio frames. Flexible subframes are also interchangeably called dynamic subframes.
Table 1 shows the existing TDD configurations (also known as UL-DL configurations or TDD UL-DL configurations).
TABLE 1UL-DL configurationsDL-to-ULSwitch-UL-DLpointSubframe numberconfigurationperiodicity012345678905 msDSUUUDSUUU15 msDSUUDDSUUD25 msDSUDDDSUDD310 ms DSUUUDDDDD410 ms DSUUDDDDDD510 ms DSUDDDDDDD65 msDSUUUDSUUD
FIG. 1a shows as an example of a dynamic TDD configuration made from two legacy TDD configurations (configuration 0 and 2). The TDD configuration is also called UL/DL subframe configuration.
Configuration of Flexible Subframes
A flexible subframe is configured in a cell and the UEs are also informed about the flexible subframes by means of signaling. A subframe is in this document referred to as flexible if it is an UL subframe in one TDD configuration and a DL or special subframe in a second TDD configuration. More specifically a subframe can be a flexible subframe if it is an UL subframe in one TDD configuration and a DL or a special subframe in another TDD configuration. A subframe can also be flexible if it is an UL subframe in one TDD configuration and a DL subframe in a second TDD configuration. The first and second TDD configurations can be used in different radio frames in the same cell or in different cells during the same or different radio frames. A TDD configuration may also interchangeably be called an UL-DL configuration or a special subframe configuration.
The two configurations may either be the configuration used for UL scheduling and Hybrid Automatic Repeat Request (HARQ) timing and the configuration used for DL HARQ timing. It could otherwise be based on fixed configurations, e.g. configuration 0 and 5 in Table 1. In this example (configuration 0 and 5) subframes {3, 4, 7, 8, 9} would be flexible.
It is currently being discussed in 3GPP that the TDD configuration that will be applied for a period of time, e.g. a period of 10, 20, 40, 80 ms, is communicated to the UE by means of DL Control Information (DCI) format 1C. The UE may need to apply this configuration from the current radio frame or in any following radio frame.
Flexible Subframes in Half Duplex Operation
In half duplex (HD), or more specifically in HD Frequency Division Duplex (HD-FDD), the UL and DL transmissions take place on different paired carrier frequencies but not simultaneously in time in the same cell. This means the UL and DL transmissions take place in different time slots or subframes. In other words UL and DL subframes do not overlap in time. The number and location of subframes used for DL, UL and subframes that are unused can vary on the basis of a radio frame or multiple radio frames. For example in one frame (say frame#1) subframes 9, 0, 4 and 5 are used for DL and subframes 2, 5 and 7 are used for UL transmission. But in another frame (say frame#2) subframes 0 and 5 are used for DL and subframes 2, 3, 5, 7, and 8 are used for UL transmission. Some subframes are unused to account for switching between UL and DL subframes. In this example subframe 3, 4, 8 and 9 can be considered as flexible subframes since they change between UL, DL and unused subframes across radio frames #1 and #2.
SI Acquisition Using Autonomous Gaps
In High Speed Packet Access (HSPA) and Long Term Evolution (LTE) the serving cell can request the UE to acquire the SI of the target cell. More specifically the SI is read by the UE to acquire the cell global identifier (CGI), which uniquely identifies the target cell.
The UE reads the SI of the target cell (e.g. intra-, inter-frequency or inter-RAT cell) upon receiving an explicit request from the serving network node via Radio Resource Control (RRC) signaling e.g. from a Radio Network Controller (RNC) in HSPA or eNode B in case of LTE. The acquired SI is then reported to the serving cell. The signaling messages are defined in the relevant HSPA and LTE specifications.
In LTE the UE has to read the master information block (MIB) and SI block#1 (SIB1) of the target Evolved Universal Terrestrial Radio Access Network (E-UTRAN) cell (which can be FDD or TDD) to acquire its CGI (also known as E-UTRAN CGI(ECGI)) when the target cell is E-UTRAN intra- or inter-frequency. The MIB and SIB1 are sent on the Physical Broadcast Channel (PBCH) and Physical Downlink Shared Channel (PDSCH) respectively over pre-defined scheduling instances.
In order to acquire the SI which contains the CGI of the target cell, the UE has to read at least part of the SI including master information block (MIB) and the relevant SI block (SIB) as described later. The terms SI reading/decoding/acquisition, CGI/ECGI reading/decoding/acquisition, CSG SI reading/decoding/acquisition are sometimes interchangeably used. For consistency the broader term “SI reading or acquisition” is used.
The reading of SI for the acquisition of CGI is carried out during measurement gaps which are autonomously created by the UE. The number of gaps and their size thus depends upon UE implementation as well as on other factors such as the radio conditions, or type of SI to be read.
For TDD intra-frequency measurements, if autonomous gaps are used for reporting CGI, the UE may be required to be able to identify a new CGI of E-UTRA cell within Tidentify_CGI, intra=Tbasic_identify_CGI, intra ms, where Tbasic_identify_CGI, intra is the maximum allowed time for the UE to identify a new CGI of an E-UTRA cell. Tbasic_identify_CGI, intra is equal to 150 ms. This requirement applies when no Discontinuous Reception (DRX) is used.
If there is continuous DL data allocation and no DRX is used and no measurement gaps are configured, then the UE shall be able to transmit at least the number of Acknowledgements/non-acknowledgements (ACK/NACKs) stated in the following Table 2 during the identification of a new CGI of a E-UTRA cell. The continuous transmission herein means that the network node transmits data in all DL subframes during the Tbasic_identify_CGI, intra.
TABLE 2Requirement on minimum number of ACK/NACKs to transmit duringTbasic—identify—CGI,intraMinimum number ofUL/DLtransmittedconfigurationACK/NACKs018135243336439542630Problem
The UE acquires the SI of a non-serving cell in autonomous gaps. During the autonomous gaps the UE does not receive and transmit in a serving cell and can thus not receive any type of serving signal including SI of the serving cell. This is because the UE can decode only one physical channel (e.g. PBCH, PDSCH) at a time, and the SI is transmitted on PBCH and PDSCH.
In existing LTE TDD solutions, the UE is required to meet pre-defined SI reading requirements, which are specified and applicable under static TDD configuration. In this case the same TDD configuration is used in all cells on the serving and non-serving carriers over the entire period (T0) during which the SI is acquired by the UE.
In order to ensure certain minimum serving cell performance the pre-defined SI reading requirements also require the UE to send at least certain number of ACK/NACK during T0 in response to continuous DL data transmission. In static TDD the HARQ timing is fixed and the requirement in terms of the number of ACK/NACK to be transmitted is also fixed and depends on the TDD configuration.
However, in a system with flexible subframe operation such as in dynamic TDD or in HD-FDD the direction of subframe can change quickly, sometimes as fast as every radio frame. In this case, the UE behavior in terms of transmitting the minimum number of ACK/NACK during T0 is unspecified. This means that with flexible subframe operation the UE will not be compliant to any requirements, thus leading to one or more of the following problems:                Degradation of serving cell performance in terms of receiving and transmitting data during the SI acquisition;        Loss of scheduling grant sent by the network node since UE may not be able to use it; and        Degradation in SI reading performance or failure to acquire the SI in case the UE creates fewer than necessary autonomous gaps during T0.        